How can i save pdf changes through Jsx in illustrator?

I stuck on saving PDF document to save changes, i don't know why illustrator is not supporting it for PDF, when i execute app.documents[0].save() , it creates an .ai file on the same path where  PDF resides. one method is known that is tosaveAs document on the same path, but i guess there should be another method to save changes in pdf file . please let me know if possible.

Thanks in advance.